1. Field of the Invention
Embodiments of this specification generally relate to wireless communication systems and more particularly to equalizing multiple-input multiple-output (MIMO) signals.
2. Description of the Related Art
Wireless communication systems may enable the transfer of data between a transmitter and multiple receivers. MIMO wireless communication systems may provide improved performance when compared to single-input-single-output systems. The improved performance may be provided by, in part, using multiple receivers to receive multiple transmitted signals. MIMO wireless communication systems may be governed, in part, by standards, such as IEEE standard (draft) 802.11n. Such standards may describe how a MIMO system receives and recovers a MIMO signal. In one embodiment, a MIMO system may be described by the equationy=H·x+n  (eq. 1)where H is a channel matrix (wherein computing the channel matrix H is well known in the art), x is a vector of input signals (hereinafter referred to as the x input signal), n is a vector of noise, and y is a vector of output signals.
A MIMO system may include multiple receivers configured to receive a MIMO signal. FIG. 1A illustrates a portion of an exemplary, prior art MIMO system 100 include “m” receive chains, wherein m is an integer equal to or greater than 2. In system 100, each receive chain 102 receives a MIMO signal via an antenna 101. Each receive chain 102 may produce a receive chain signal, denoted as yi in FIG. 1A. In general, a MIMO system may equalize the outputs of the multiple receive chains to recover the x input signal. One way to recover the x input signal in a MIMO system uses a Minimum Mean Square Error (MMSE) detector. The MMSE detector may be described by the equationx=(H*H+σ2I)−1H*·y  (eq. 2)where σ2 is a noise power value and I is the identity matrix. Oftentimes, recovering the x input signal may be referred to as equalizing y and (H*H+σ2I)−1H* may be referred to as the equalization coefficient. Equation 2 assumes that the noise power value is the same on all receive chains.
As shown in FIG. 1A, equalization may occur in an equalizer 103, which receives the output signal vectors y, from the multiple receive chains 102. However this approach has disadvantages. Specifically, if one or more of receive chains have relatively noisy receive chain signals (and therefore are of lower relative quality), and these receive chains signals are equally weighted with receive chains signals that have relatively lower amounts of noise (and are therefore of higher relative quality) during equalization, then the overall performance of the MIMO system is degraded because the equalized signal is of relatively lower quality.
As equation 2 above illustrates, a noise power value is typically required to recover the x input signal when using an MMSE detector. The noise power value may be determined by periodically measuring the receive power of a receive chain 102 during a quiet period, i.e. a period when no signals are being received. Because no signals are being received, the resulting measurement represents noise as seen by the receive chain 102. Typically, the receive power may be determined by examining the output of an analog front end, squaring that output, and then averaging the squares.
FIG. 1B illustrates an exemplary receive chain 102 including an analog front end 111 and a digital signal processing unit 115. Analog front end 111 includes a variable gain amplifier (VGA) 112, an analog to digital converter (ADC) 113, and an automatic gain controller (AGC) 114. Digital signal processing unit 115 includes a Fast Fourier Transform (FFT) unit 116. As shown, the input to VGA 112 may be provided by antenna 101. The output of VGA 112 may be provided to ADC 113. The output of ADC 113 may be provided to both AGC 114 and FFT unit 116. AGC 114 determines the gain of VGA 112 by examining, among other things, the output of ADC 113. The output of AGC 114 is provided to VGA 112. The output of FFT unit 116 produces the receive chain signal yi.
As described above, noise power value may be determined by measuring receive power during a quiet period. Such a measurement may be implemented using analog front end 111 of receive chain 110. For example, during a quiet period, the receive power may be determined by measuring, squaring, and averaging the outputs of ADC 113. Often, AGC 114 may set the gain of VGA 112 to a value such that the output of ADC 113 (and therefore the noise power value) is not limited by the upper or lower bounds of ADC 113.
Therefore, a noise power measurement may be associated with a particular VGA gain setting. As is well-known, a noise power measurement may be scaled using the VGA gain setting information to represent the noise power present at the input of the analog front end. This is often referred to as a root-mean-square (RMS) noise power value. Determining noise power in this way advantageously uses relatively smaller amounts of hardware and relatively smaller amounts of computation and in doing so produces a noise power value relatively quickly and is ready to use when receiving an actual receive chain signal.
Determining a noise power value in the above manner, however, has several disadvantages. A first disadvantage is that such a noise power value may not include several sources of noise. For example, although the method may capture thermal noise power, the method may not capture phase noise, quantization error, or distortion. This is because these other noise sources may be determined by measuring an actual receive chain signal in operation, not by measuring receive power during a quiet period.
A second disadvantage is that the accuracy of the noise power value may depend upon the linearity of the VGA. This is because the noise power value may be scaled in accordance with a gain setting within the VGA.
A third disadvantage is that the power measurement path (which includes the analog front end of a receive chain) may be different from the data path (which includes the analog front end and FFT unit of a receive chain). Although this may be overcome by scaling the determined noise power value by any signal gains or losses that may occur in the data path relative to the power measurement path, scaling is undesirable because it must be re-examined each time new hardware is developed (i.e. hardware may significantly affect the scaling factor).
Therefore, a noise power value determined in the manner described above may be inaccurate. As a result, the performance of MIMO equalizer 103 (FIG. 1A) receiving the multiple receive chain signals y, may be degraded.
Therefore, what is needed is a way to improve the recovery of input signals transmitted in a MIMO system, especially when one or more of the received MIMO signals may be relatively noisy.